This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of a water-external microemulsion comprised of an aqueous medium containing neutralized organic acids extracted from a petroleum oil.
In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding techniques. To data one of the more promising low tension waterflooding techniques involves the injection of aqueous solutions of petroleum sulfonates within a designated equivalent weight range and under controlled conditions of salinity. For example, in a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", JOURNAL OF PETROLEUM TECHNOLOGY, Vol. 25, Feb. 1973, pp. 205-210, there is disclosed a procedure which involves the sequential injection of a protective slug, a surfactant slug, and a mobility control slug. The protective slug is an aqueous solution of sodium chloride which is injected in order to displace the reservoir water ahead of the subsequently injected surfactant slug. This slug is substantially free of divalent ions which would tend to precipitate the surfactant slug. It, as well as the surfactant slug, may contain inorganic sacrificial agents such as sodium carbonate and/or sodium tripolyphosphate which function to reduce adsorption of the surfactant.
The surfactant slug comprises an aqueous solution of petroleum sulfonates having an average molecular weight within the range of 350-500 and contains sodium chloride in a concentration, typically about 1.0 to 2.0 weight percent, which will promote the desired low interfacial tension between the injected water and the reservoir oil. The subsequently injected thickened water slug contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide an initial viscosity greater than viscosity of the reservoir oil and a terminal viscosity near that of water. Thereafter a driving fluid such as produced field brine is injected in order to carry the process to conclusion.
Another waterflooding procedure in which the displacement of oil by the injected water is enhanced by a low oil-water interfacial tension is disclosed in U.S. Pat. No. 3,929,190 Harry L. Chang, Evin L. Cook, and Ralph F. Burdyn. In this process, a petroleum oil is contacted with an aqueous solution of an alkaline agent in order to extract neutralized organic acids from the oil into the aqueous medium. The resulting aqueous solution of neutralized organic acids, which may contain some oil in the form of a fine emulsion, is then injected into the reservoir to displace the oil therein.
Another approach to the enhanced recovery of oil involves the injection of a water-oil-surfactant system referred to generally as a "microemulsion" system. This system is defined by Healy et al., "Physicochemical Aspects of Microemulsion Flooding", SPE Paper 4583, 48th Annual Fall Meeting of the SPE of AIME, Las Vegas, Sept. 30-Oct. 3, 1973, as a "stable, translucent micellar-solution of oil, water that may contain electrolytes, and one or more amphiphilic compounds (i.e., surfactants, alcohols, etc.)".
A somewhat different approach to defining these systems is taken in U.S. Pat. No. 3,506,071 to Jones. The patentee here refers to the term "micellar dispersion" as including "micellar solutions", "water-external `micro-emulsion`", and "`transparent` emulsions". Jones suggests that the strongest differentiation between micellar dispersions and emulsions is the thermodynamic stability of the former.
Regardless of the terminology used, such microemulsions generally contain, in addition to water and oil, surfactants such as petroleum sulfonates, co-surfactants such as alcohols, and electrolytes. For example, the aforementioned patent to Jones refers to the volume amounts of these various constituents as being about 1 percent to about 50 percent of hydrocarbon, from about 40 percent to about 95 percent aqueous medium, at least about 4 percent sulfonate, from about 0.01 percent to about 5 percent of semipolar compound (co-surfactant), and up to about 4 percent by weight of electrolyte. Electrolytes which are said to be useful in Jones include inorganic bases such as sodium hydroxide, salts such as sodium chloride, and inorganic acids such as hydrochloric acid. The patentee notes that acids and neutral salts are preferred when the system contains hydrophilic sulfonates and where high reservoir temperatures are involved, whereas electrolytes such as sodium hydroxide which yield a higher pH are preferred with the more oleophilic sulfonates.